Production of phthalic anhydride from phthalic acid

ABSTRACT

Melt of phthalic anhydride containing a small amount of azeotroping agent is maintained at a temperature of from 160* to 200*C. Phthalic acid is introduced into the melt, and an azeotropic mixture of water and azeotroping agent is withdrawn therefrom. The withdrawn azeotroping agent is separated from the water in the azeotropic mixture and returned to said melt.

Waited States Patent n91 List et ai.

[ 1 PRODUCTION OF PHTHALHC ANHYDRIDE FROM PHTHALIC ACID [75] Inventors:Ferdinand List; Helmut Alfs, both of Marl, Germany [73] Assignee:Chemische Werke Huls gesellschait, Marl, Germany 221 Filed: Dec. 2, 1969211 Appl.No.: 881,598

Aktien- [30] Foreign Application Priority Data Dec. 7, 1969 Germany ..P18 13 391.0

[52] US. Cl ..260/346.7 [51] ..C07c 63/18 [58] Field of Search..260/346.7, 346.4, 346.3

[5 6] References Cited UNITED STATES PATENTS 2,925,425 2/1960 Contois etal. ..26(l/346.4

I 1March 13, 1973 3,161,658 12/1964 Meyer ..260/346.3 3,137,660 6/1964Jones ..260/2.2

OTHER PUBLICATIONS Ono et al., Chem. Abstr. (1938) Vol. 32, p. 41433.

Primary Examiner-Alex Mazel Assistant Examiner-Bernard DentzAttorney-Krafft & Wells [57] ABSTRACT 8 Claims, N0 Drawings PRODUCTIONOF PHTHALIC ANHYDRIDE FROM PnTnKuc ACID BACKGROUND or THE INVENTION Forprocessing, e.g., esterification with various alcohols, o-phthalic acid(PA) is advantageously employed in the form of its anhydride (PAN). WhenPA is heated (to form PAN) without the addition of an azeotroping agent,the amount of water liberated is inadequate to produce the desiredproduct. Moreover, a large proportion of the thus-formed water ofreaction runs back into the formed melt, resulting in a reformation ofPA. The conversion is low and the produced PAN has a high acid number.The sublimation of PAN also presents difficulties in this mode ofoperation by clogging the withdrawal lines.

Attempts to overcome these difficulties by the use of conventionalazeotropic agents, such as toluene,

' diethylbenzene, cumene and diisopropylbenzene, yield onlyunsatisfactory results. Although, under these conditions, obstructionsby sublimate in the gas and condensation chamber are avoided, thereaction rate is too low, and too high a concentration of free acidremains in the melt. Obviously, the temperature of the melt, due to theformation of the azeotropic mixture, is reduced to such an extent thatthe production of PAN can take place only very sluggishly.

Consequently, there is need for a commercially feasi-' ble process toconvert ortho-phthalic acid readily and completely into the anhydride onan industrial scale.

Although there are fundamentally different conventional methods forpreparing PA (Kirk-Othmer, Encyclopedia of Chemical Technology." 2ndEdition, Vol. 15, page 450, (1968), and the conversion of PA to PAN isnot dependent upon the method according to which the PAwas prepared,there is an additional advantage to the instant method for preparing PANwhen the PA starting material is conventionally obtained by oxidizingo-xylene in the liquid phase, for example with air, in glacial aceticacid in the presence of a bromineactivated soluble heavy metal catalystsystem.

SUMMARY A melt of PAN and a small amount of azeotroping agent isprepared and maintained at an elevated temperature. PA is introducedinto the melt; azeotroping agentand water of reaction entrained therebyare withdrawn from said melt. After separating the withdrawn water fromthe withdrawn azeotroping agent, the latter is returned into the melt.Additional PA can be introduced into the melt either from time to timeor continuously; likewise, PAN can be withdrawn from the meltintermittently or continuously.

DETAILS Although continuous operation is practical and is readilyeffected, it is preferred to introduce the PA into the melt initially inbatches. The melt is maintained at a temperature of from 160 to 200C,and preferably from 170 to 185C.

The azeotroping agent is any known hydrocarbon which forms an azeotropewith water. Examples are benzene, toluene, ethylbenzene, o-xylene,m-xylene, pxylene, cumene, diethylbenzene, diisopropylbenzene and amixture of any combination of two or more of the foregoing.

0 azeotroping agent. Advantageously, the amount of azeotroping agent inthe reaction chamber is from 5 to 40, preferably from 10 to 25 percentby weight, based on the weight of the PAN present.

Of the illustrated azeotroping agents, it is particularly advantageousto employ o-xylene, even though the boiling point of this compound isonly 144C.

The utilization of this hydrocarbon affords the advantage that theoxidation of o-xylene to o-phthalic acid and the conversion of thelatter to phthalic anhydride can be conducted in successive processstages without introducing any foreign substance into the total process.The o-xylene can be directly recycled into the oxidation stage.

Although the amount of PA introduced into the melt can be as much as 25times the weight of PAN in the melt, said amount is advantageouslymaintained below 10, preferably 4 to 8, especially about 6 times saidweight.

The concentration of PA in the PAN melt can vary within wide ranges dueto the fact that the PA immediately after its addition begins to react,and this reaction comes to an end if no fresh PA is added then.

Without further elaboration, it is believed that one skilled in the artcan, using the preceding description, utilize the present invention toits fullest extent.

EXAMPLE 1.

250 kilograms of phthalic anhydride are melted in a vessel having astirrer and heated to 170C. The stirred vessel is equipped with a refluxcondenser and a water trap. 50 kg. of o-xylene are introduced into themelt vessel and, in parallel therewith, 1.6 (metric) tons of ophthalicacid are also gradually added. At C, a very strong watercleavage occurs.The water of reaction, which is withdrawn overhead together witho-xylene, is separated in the trap and discarded, whereas thexyleneflows back into the stirred vessel. After a total production of kg. of H0, the dehydration is terminated. The dehydrating period is 20 minutes.The thus-obtained crude phthalic anhydride exhibits the followinganalytical values:

Acid number: Melting point:

735 (theory: 757) 127C (literature: 131C) vThe yield is practicallyquantitative.

Replacing the o-xylene with 50 kg. of either benzene, toluene,ethylbenzene, diethylbenzene, cumene, diisopropylbenzene or a commercialmixture of mand p-xylene yields essentially the same results.

EXAMPLE 2.

PA is prepared by liquid phase air oxidation of oxylene in acetic acid(as solvent) with a bromine-activated heavy metal catalyst system(Kirk-Othmer, supra, p. 451). Resulting PA (2490 kg.) and unconvertedo-xylene are gradually introduced together into a PAN melt (as describedin Example 1) without the addition of any further o-xylene or otherazeotroping agent. As the water of reaction is removed with oxylene, itis separated from the latter, the thus separated o-xylene is recycledfor the production of further PA, which is introduced with unconvertedoxylene into the PAN melt. As the PAN melt increases in volume, some isdrawn off to maintain an essentially fixed level. The conversion ofo-xyene to PAN is thus effected continuously.

We claim:

1. A process for preparing phthalic anhydride by splitting off waterfrom phthalic acid which comprises a. introducing phthalic acid into amelt of phthalic anhydride containing in contact therewith a smallamount of a hydrocarbon azeotroping agent sufficient to form anazeotrope with the water split off from said phthalic acid, the meltbeing maintained at a temperature of from 160 to 200C,

b. withdrawing water and azeotroping agent from said melt,

c. separating withdrawn water from the azeotroping agent, and

d. replacing in contact with said melt azeotroping agent approximatingthe amount withdrawn therefrom.

2. A process for preparing phthalic anhydride comprising:

a. preparing a melt of phthalic anhydride containing in contacttherewith about 5 to 40 percent by weight of a hydrocarbon azeotropingagent which forms an azeotrope with water based on the weight of saidphthalic anhydride and maintaining said melt at a temperature of aboutto 200C;

. introducing phthalic acid into said melt and converting to phthalicanhydride by splitting off water; withdrawing said water and a portionof said azeotroping agent from said melt;

separating said water from said portion of azeotroping agent; andreplacing approximately said portion of azeotroping agent in contactwith said melt.

3. A process according to claim 2, wherein said portion of azeotropingagent is recycled to contact with said melt.

4. A process according to claim 2, wherein a portion of the phthalicanhydride melt is withdrawn as more phthalic anhydride is produced.

5. A process according to claim 2, wherein the azeotroping agent is amember selected from the group consisting of benzene, toluene,ethylbenzene, diethylbenzene, cumene, diisopropylbenzene, o-xylene,mxylene, p-xylene and a mixture of at least two of any of the foregoing.

6. A process according to claim 5, wherein the azeotroping agent iso-xylene.

7. A process according to claim 6, wherein said portion of azeotropingagent is recycled to a liquid phase preparation of phthalic acid fromo-xylene and resulting phthalic acid is introduced with associatedunconverted o-xylene into the phthalic anhydride melt.

8. A process according to claim 3, wherein the phthalic acid isintroduced batchwise into the phthalic anhydride melt.

t i l i l

1. A process for preparing phthalic anhydride by splitting off water from phthalic acid which comprises a. introducing phthalic acid into a melt of phthalic anhydride containing in contact therewith a small amount of a hydrocarbon azeotroping agent sufficient to form an azeotrope with the water split off from said phthalic acid, the melt being maintained at a temperature of from 160* to 200*C, b. withdrawing water and azeotroping agent from said melt, c. separating withdrawn water from the azeotroping agent, and d. replacing in contact with said melt azeotroping agent approximating the amount withdrawn therefrom.
 2. A process for preparing phthalic anhydride comprising: a. preparing a melt of phthalic anhydride containing in contact therewith about 5 to 40 percent by weight of a hydrocarbon azeotroping agent which forms an azeotrope with water based on the weight of said phthalic anhydride and maintaining said melt at a temperature of about 160* to 200*C; b. introducing phthalic acid into said melt and converting to phthalic anhydride by splitting off water; c. withdrawing said water and a portion of said azeotroping agent from said melt; d. separating said water from said portion of azeotroping agent; and e. replacing approximately said portion of azeotroping agent in contact with said melt.
 3. A process according to claim 2, wherein said portion of azeotroping agent is recycled to contact with said melt.
 4. A process according to claim 2, wherein a portion of the phthalic anhydride melt is withdrawn as more phthalic anhydride is produced.
 5. A process according to claim 2, wherein the azeotroping agent is a member selected from the group consisting of benzene, toluene, ethylbenzene, diethylbenzene, cumene, diisopropylbenzene, o-xylene, m-xylene, p-xylene and a mixture of at least two of any of the foregoing.
 6. A process according to claim 5, wherein the azeotroping agent is o-xylene.
 7. A process according to claim 6, wherein said portion of azeotroping agent is recycled to a liquid phase preparation of phthalic acid from o-xylene and resulting phthalic acid is introduced with associated unconverted o-xylene into the phthalic anhydride melt. 